1. Field of the Invention
This invention relates to a variable-reluctance electric driving motor comprising a stator ring made up of magnetic laminations and provided with poles surrounded by induction coils and distributed in four groups of the same number, a rotor ring made also of magnetic laminations and provided with teeth of uniform pitch cooperating with said poles.
The relative positions of the poles of the stator with respect to the rotor teeth are shifted by 1, 2, 3, 4 quarters of the tooth pitch or a multiple thereof in respectively the first, second, third, fourth group, induction coils surrounding the poles of each group. Means are provided for feeding the induction coil means of each group with a four-phase current to obtain the magnetic saturation of each pole when, during the rotation of the rotor, at least a portion of a tooth is facing said pole.
In order to obtain the four-phase current from a D.C. source the feeding means comprise switching means controlled by the position of the rotor and connected to the induction coil means so that, during a cycle corresponding to any one rotor advance through a tooth pitch, each stator pole is subjected to two supply switching operations in opposite directions, said switching operations being offset by a quarter cycle from one group to the next.
2. Description of the prior art.
U.S. Pat. No. 3,435,266 issued Mar. 25, 1969 to the present Applicants, in particular, has already disclosed a first motor of the kind concerned in which the stator poles, provided with individual coils, are spread out with a constant pitch so that four consecutive poles belong to the said four groups respectively, and in which the rotor teeth are of an odd multiple number of one-quarter of that of the stator poles.
U.S. Pat. No. 3,740,630 issued June 19, 1973 to the present Applicants also discloses a second motor of the kind in which the stator comprises a number 4 m (m.gtoreq.2) of identical sectors, in each of which the stator poles of number p are uniformly distributed with the pitch of the rotor teeth and grouped together by a single sector coil, two consecutive sectors being spaced apart by an angle such that from one to the other the teeth of the rotor recede (with respect to the direction of movement) by one-quarter of the said tooth pitch or an odd multiple n of said quarter-pitch. It will readily be seen that, in this case, the number of rotor teeth is m (4 p+n).
The characteristics of the variable-reluctance electric motors of these two patents can be summarized as follows:
______________________________________ U.S. Pat. U.S. Pat. No. 3,435,266 No. 3,740,630 ______________________________________ Number of 4m m sectors Number of stator 4mp 4m poles Number of rotor teeth ##STR1## mn The pole pitch is The pole pitch constant per sector is constant all and equal to the tooth around the stator. pitch and the sectors There is a winding are specially phase per pole. shifted with respect to one another. There is a winding per sector. ______________________________________
In the present invention the pole pitch is not constant in the sectors.
In the motors of both types, the saturation means for the magnetic circuits are based on the fact that each rotor tooth has, in the peripheral direction and as from the air gap, a central region of maximum magnetic material density in radial section symmetrically framed by two marginal zones in which the said magnetic material density decreases progressively generally discontinuously towards the leading and trailing edges of the tooth.
This procedure has the effect of "modulating" the effective back emf in each stator coil per unit of time in the manner indicated hereinabove.
However, it has been found that the presence of a magnetically saturable zone in the toothed part of the rotor adjacent the air gap but situated outside the stator zone carrying the coils results in the formation of leakage flux, and this results in some limitation of the flux variations inside these coils and hence reduced power and efficiency.